
<h2>DesignForExtension</h2>

<div>
<h3><a id="Description"></a>Description</h3>

<p>
          The check finds classes that are designed for extension (subclass creation).
        </p>

<p>
          Nothing wrong could be with founded classes.
          This check makes sense only for library project (not an application projects)
          which care of ideal OOP-design to make sure that class works in all cases even misusage.
          Even in library projects this check most likely will find classes that are designed
          for extension by somebody. User needs to use suppressions extensively to got a benefit from
          this check, and keep in suppressions all confirmed/known classes that are deigned for
          inheritance intentionally to let the check catch only new classes, and bring this to
          team/user attention.
        </p>

<p>
         ATTENTION: Only user can decide whether a class is designed for extension or not.
         The check just shows all classes which are possibly designed for extension.
         If smth inappropriate is found please use suppression.
        </p>

<p>
         ATTENTION: If the method which can be overridden in a subclass has a javadoc comment
         (a good practise is to explain its self-use of overridable methods) the check will not
         rise a violation. The violation can also be skipped if the method which can be overridden
         in a subclass has one or more annotations that are specified in ignoredAnnotations
         option. Note, that by default @Override annotation is not included in the
         ignoredAnnotations set as in a subclass the method which has the annotation can also be
         overridden in its subclass.
        </p>

<p>
          Problem is described at "Effective Java, 2nd Edition by Josh Bloch" book, chapter "Item 17: Design and document for inheritance or else prohibit it".
        </p>

<p>
          Some quotes from book:
        </p>

<blockquote>The class must document its self-use of overridable methods.
By convention, a method that invokes overridable methods contains a description
of these invocations at the end of its documentation comment. The description
begins with the phrase “This implementation.”
        </blockquote>

<blockquote>The best solution to this problem is to prohibit subclassing in classes that
are not designed and documented to be safely subclassed.
        </blockquote>

<blockquote>If a concrete class does not implement a standard interface, then you may
inconvenience some programmers by prohibiting inheritance. If you feel that you
must allow inheritance from such a class, one reasonable approach is to ensure
that the class never invokes any of its overridable methods and to document this
fact. In other words, eliminate the class’s self-use of overridable methods entirely.
In doing so, you’ll create a class that is reasonably safe to subclass. Overriding a
method will never affect the behavior of any other method.
        </blockquote>

<p>
          The check finds classes that have overridable methods (public or protected methods
          that are non-static, not-final, non-abstract) and have non-empty implementation.
        </p>


<p>
          Rationale: This library design style protects superclasses against
          being broken by subclasses. The downside is that subclasses are
          limited in their flexibility, in particular they cannot prevent
          execution of code in the superclass, but that also means that
          subclasses cannot corrupt the state of the superclass by forgetting
          to call the superclass's method.
        </p>

<p>
          More specifically,
          it enforces a programming style where superclasses provide empty
          "hooks" that can be implemented by subclasses.
        </p>

<p>
          Example of code that cause violation as it is designed for extension:
        </p>

<div class="source">
<pre>public abstract class Plant {
    private String roots;
    private String trunk;

    protected void validate() {
      if (roots == null) throw new IllegalArgumentException("No roots!");
      if (trunk == null) throw new IllegalArgumentException("No trunk!");
    }

    public abstract void grow();
}

public class Tree extends Plant {
    private List leaves;

    @Overrides
    protected void validate() {
      super.validate();
      if (leaves == null) throw new IllegalArgumentException("No leaves!");
    }

    public void grow() {
      validate();
    }
}
        </pre></div>

<p>
          Example of code without violation:
        </p>

<div class="source">
<pre>public abstract class Plant {
    private String roots;
    private String trunk;

    private void validate() {
        if (roots == null) throw new IllegalArgumentException("No roots!");
        if (trunk == null) throw new IllegalArgumentException("No trunk!");
        validateEx();
    }

    protected void validateEx() { }

    public abstract void grow();
}
        </pre></div>
      </div>


<div>
<h3><a id="Properties"></a>Properties</h3>

<table class="bodyTable">

<tbody><tr class="a">

<th>name</th>

<th>description</th>

<th>type</th>

<th>default value</th>
                </tr>

<tr class="b">

<td>ignoredAnnotations</td>

<td>
                        Annotations which allow the check to skip the method from validation.
                    </td>

<td>String Set</td>

<td><code>Test, Before, After, BeforeClass, AfterClass</code></td>
                </tr>
            </tbody></table>
        </div>


<div class="section">
<h3><a id="Examples"></a>Examples</h3>

<p>
          To configure the check:
        </p>


<div class="source">
<pre>&lt;module name="DesignForExtension"/&gt;
        </pre></div>


<p>
          To configure the check to allow methods which have @Override and @Test annotations to be
          designed for extension.
        </p>


<div class="source">
<pre>&lt;module name="DesignForExtension"&gt;
  &lt;property name="ignoredAnnotations" value="Override, Test"/&gt;
&lt;/module&gt;
        </pre></div>


<div class="source">
<pre>public class A extends B {
  @Override
  public int foo() {
    return 2;
  }

  public int foo2() {return 8;} // violation
}

public class B {
  /**
   * This implementation ...
     @return some int value.
   */
  public int foo() {
    return 1;
  }

  public int foo3() {return 3;} // violation
}

public class FooTest {
  @Test
  public void testFoo() {
     final B b = new A();
     assertEquals(2, b.foo());
  }

  public int foo4() {return 4;} // violation
}
        </pre></div>
      </div>



